Speaker: Tom Wier
Connectivity is worse, slower, and expensive and hence unavailable to most people So take care to send less data - overall! Single page applications might work better since there is a large first request, but only small subsequent requests. The single page applications gives you more control in working around slow connections, too. Consider carefully if this is worth it.
Languages and literacy
Less people speak English. Internationalize your website. Support major languages at least, but start thinking about smaller local languages, too. Use gettext. Use translation comments explaining the context. Pay attention to pluralization, and add context strings (pgettext).
Incomplete or wrong data
Addresses or locations work differently. See also: falsehoods programmers believe about addresses. You can also use fuzzy input to match places that have no consistent spelling.